Semiconductor laser device and method for fabricating the same

ABSTRACT

A method for fabricating a buried semiconductor laser device including the steps of: forming a mesa structure including a bottom cladding layer, an active layer and a top cladding layer overlying an n-type semiconductor substrate; and forming a current confinement structure by growing a p-type current blocking layer and an n-type current blocking layer on each side surface of the mesa structure and on a skirt portion extending from the each side surface, the p-type current blocking layer being fabricated by using a raw material gas containing a group III element gas and a group V element gas at a molar ratio between 60 and 350 inclusive. In this method, the semiconductor laser device including the current confinement structure with the specified leakage current path width can be fabricated with the excellent reproducibility.

RELATED APPLICATIONS

[0001] The present application claims benefit of priority under 35U.S.C. §119 to the following Japanese patent Applications Nos.2000-364387 (filed on Nov. 30, 2000) and 2001-360940 (filed on Nov. 27,2001), each of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] (a) Field of the Invention

[0003] The present invention relates to a semiconductor laser device anda method for fabricating the same, and more particularly to a so-calledburied semiconductor laser device having a higher laser emissionefficiency and a higher reproducibility of a current-optical outputcharacteristic.

[0004] (b) Description of the Related Art

[0005] A semiconductor laser device having a lower threshold currentdensity and a higher laser emission efficiency is desirable. A strainedquantum well semiconductor laser device having a hetero-structure and apair of current blocking layers is attracting public attention becauseof the excellent characteristics thereof. The semiconductor laser havinga pair of current blocking layers in abutment to the semiconductor laserstructure is generally called a buried semiconductor laser.

[0006] A conventional strained quantum well semiconductor laser deviceshown in JP-A-8(1996)-288589 will be described referring to FIG. 1A.

[0007] As shown in idealized form in FIG. 1A, a conventional strainedquantum-well semiconductor laser device 20 includes a layer structurehaving an n-type InGaP bottom cladding layer 2, an active layer 3, and ap-type InGaP top cladding layer 4, sequentially and epitaxially grown onan n-type GaAs substrate 1 by using a metal organic chemical vapordeposition (MOCVD) method.

[0008] The active layer 3 is a five-layered structure including anInGaAsP layer 5, a GaAs layer 6, an InGaAs layer 7, a GaAs layer 8 andan InGaAsP layer 9.

[0009] The top cladding layer 4, the active layer 3 and the top part ofthe bottom cladding layer 2 are configured to have a mesa structure 11.Each of the side surfaces 12 of the mesa structure 11 and the adjacentsurfaces of the bottom cladding layer 2 are covered with a p-type InGaPcurrent blocking layer 14 and an n-type InGaP current blocking layer 15,which are sequentially deposited.

[0010] A second p-type InGaP top cladding layer 16 and a p-type contactlayer 17 are sequentially deposited on the n-type InGaP current blockinglayer 15, the p-type InGaP current blocking layer 14 and the topcladding layer 4 of the mesa structure 11.

[0011] A p-side metal electrode layer 18 and an n-side metal electrodelayer 19 are deposited on the top surface of the p-type contact layer 17and the bottom surface of the substrate 1, respectively.

[0012] The above publication points out a problem when the p-typecurrent blocking layer 14 and the n-type current blocking layer 15 aregrown by using an etching mask. Referring to FIG. 1B, structural defectssuch as hollows and grooves 40 are formed on the n-type current blockinglayer 15 along the bottom surface of the etching mask due to thedifference between the growth rates.

[0013] When the hollows 40 on the n-type current blocking layer 15 arelarge, crystal dislocations are liable to occur along the lines 41 shownin FIG. 1B. The propagation of a crystal dislocation from a point withinlayer 15 to a point within the p-type contact layer 17 increases thethreshold current of the fabricated laser device, which lowers the laseremission efficiency.

[0014] The above publication describes the growth conditions of thep-type and n-type current blocking layers 14, 15 such that the substratetemperature is between 750° C., and 800° C. and a mixing ratio(concentration ratio) of a group V element gas with respect to a groupIII element gas is between 400:1 and 800:1 inclusive (V:III), therebysuppressing the occurrence of the structural defects (e.g., hollows) todecrease the probability and magnitude of the crystal dislocations. (Asused later herein, we will abbreviate the conventional notation for theV:III chemical ratios from 400:1 to simply read as “400,” which meansthe molar amount of the group V element gas divided by the molar amountof group III element gas).

[0015] Since the disappearance of the structural defects thickens then-type current blocking layer 15 in the vertical direction formedoverlying the substrate 1, the amount of leakage current flowing throughthe current blocking layers 14, 15 is decreased, which in turn increasesthe laser emission efficiency when a voltage is applied between theelectrodes 18, 19.

[0016] Further, Mitsubishi Denki Giho (Mitsubishi Electric Advance) Vol.67, No. 8 (1993), p. 88 points out a decrease of the laser emissionefficiency due to a leakage current which does not contribute to thelaser emission and which flows along the interface between the mesastructure and the current blocking layer.

[0017] The buried semiconductor laser device with the reduced leakagecurrent includes higher laser emission efficiency, good linearities ofthe higher output characteristic, and an excellent current-voltagecharacteristic. Accordingly, when the leakage current path width isreduced, the resistance of the current blocking layer increases toprovide desirable laser characteristics.

[0018] Even when the current blocking layer is formed under theconditions described in the former publication such that the substratetemperature is between 750° C. and 800° C., and the mixing ratio betweenthe group V element gas and the group III element gas is between 400 and800, the leakage current path width is quite difficult to be formed in anarrower manner with the excellent reproducibility, and the values ofthe widths are difficult to be regulated and controlled.

[0019] Similarly, in the fabrication of the buried semiconductor laserdevice formed on the p-type substrate, an n-type InP contact layer isexcessively grown to be in contact with an n-type InP contact layer, anda leakage current path width is increased.

[0020] As a result, the increased leakage current lowers the laseremission efficiency to worsen the output characteristic and thelinearity of the current-voltage characteristic, and the buriedsemiconductor laser device with the higher output can be hardlyfabricated with the excellent reproducibility.

SUMMARY OF THE INVENTION

[0021] The present invention encompasses buried semiconductor laserdevices and methods of manufacturing the same. An exemplary generalmethod according to the present invention comprising forming a mesastructure including a bottom cladding layer, an active layer and a topcladding layer overlying a semiconductor substrate. The mesa structurehas at least one side surface extending from the top surface of the mesatoward the bottom cladding layer, with the active layer having anexposed side thereat. The mesa structure also has a skirt surfaceextending outward from each side surface to cover a portion of thesubstrate's surface. The exemplary general method further comprisinggrowing a first current-confinement layer on the mesa's at least oneside surface, with the first current-confinement layer comprising asemiconductor material and having a first conductivity type (e.g.,p-type or n-type). A second current-confinement layer is then grownabove at least a portion of the first current-confinement layer, thesecond current-confinement layer comprising a semiconductor material andhaving a second conductivity type which is opposite to the firstconductivity type. The closest spacing distance between the secondcurrent-confinement layer and the active layer defines a “leakagecurrent path width” (e.g., Tn or Tp). This spacing distance is normallyshown in a cross-sectional plane which is perpendicular to the topsurface of the substrate, and which is oriented to provide the smallestwidth of the mesa. The first confinement layer is grown at a temperatureranging from 610° C. to 700° C. using a raw material gas comprising agroup V element gas and a group III element gas at a molar ratio of thegroup V element gas with respect to the group III element gas having avalue between 50 and 500, inclusive, to provide a value of the leakagecurrent path width ranging from 0.15 μm to 0.60 μm.

[0022] As used herein, the term “group V element gas” is defined asincluding any precursor gas comprised of molecules, each molecule of theprecursor gas comprising one or more atoms of an element listed in thefifth column of the Periodic Table. The term “group III element gas” isdefined as including any precursor gas comprised of molecules, eachmolecule of the precursor gas comprising one or more atoms of an elementlisted in the third column of the Periodic Table. A raw material gas mayalso comprise precursor gases which carry dopant atoms (e.g., elementsin the fourth and sixth columns of the Periodic Table). A group Velement gas may comprise two or more different precursor gases (eachcarrying atoms in the fifth column of the Periodic Table), such thecombination of a Phosphorous carrying precursor gas and an Arseniccarrying precursor gas. Likewise, a group III element gas may comprisetwo or more different precursor gases (each carrying atoms in the thirdcolumn of the Periodic Table), such the combination of an Indiumcarrying precursor gas and an Gallium carrying precursor gas.

[0023] In one exemplary implementation of the present invention(generally described below under “Embodiment 1”), a buried semiconductorlaser device is fabricated using the steps of: forming a mesa structureincluding a bottom cladding layer, an active layer and a top claddinglayer overlying an n-type semiconductor substrate; and forming a currentconfinement structure by growing a p-type current blocking layer (firstcurrent confinement layer) and an n-type current blocking layer (secondcurrent confinement layer) on each side surface of the mesa structureand preferably on each skirt portion extending from each correspondingside surface. The p-type current blocking layer is fabricated by using araw material gas comprising a group V element gas and a group IIIelement gas at a molar ratio of the group V element gas with respect tothe group III element gas between 50 and 500 inclusive to provide avalue of the leakage current path width ranging from 0.15 μm to 0.60 μm.

[0024] In another exemplary implementation of the present invention(generally described below under “Embodiment 2”), a buried semiconductorlaser device is fabricated using the steps of: forming a mesa structureincluding a bottom cladding layer, an active layer and a top claddinglayer overlying a p-type semiconductor substrate; and forming a currentconfinement structure by growing a p-type separation layer (firstcurrent confinement layer), an n-type current blocking layer (secondcurrent confinement layer), and a p-type current blocking layer (thirdcurrent confinement layer) on each side surface of the mesa structureand on preferably on each skirt portion extending from eachcorresponding side surface. The p-type separation layer is fabricated byusing a raw material gas comprising a group V element gas and a groupIII element gas at a molar ratio of the group V element gas with respectto the group III element gas between 50 and 500 inclusive, to provide avalue of the leakage current path width ranging from 0.15 μm to 0.60 μm.

[0025] In accordance with the present invention, the suitable selectionof the molar ratio of the group V element gas with respect to the groupIII element gas suppresses the structural defects such as the hollowsand the trenches on the surface of the buried layer, and the buriedsemiconductor laser device including the current confinement structurehaving the specified leakage current path width can be fabricated withexcellent reproducibility and higher yield.

[0026] The buried semiconductor laser device according to the presentinvention also has larger laser emission efficiency, reduced leakagecurrent, higher optical output, excellent linearity (lack of kinks) ofthe output characteristic with respect to driving current, and goodlinearity of the current-voltage characteristic in the lasing region ofdevice operation.

[0027] The above and other objects, features and advantages of thepresent invention will be more apparent from the following description.

BRIEF DESCRIPTION OF DRAWINGS

[0028]FIG. 1A is an idealized schematic sectional view of a conventionalburied semiconductor laser device according to the prior art.

[0029]FIG. 1B is a sectional view of a buried semiconductor laser deviceaccording to the prior art showing where hollows and crystaldislocations commonly occur with fabrication method of the prior art.

[0030] FIGS. 2A-2C are graphs showing various relations between molarratios of raw material gases, growth temperatures, and the leakagecurrent path width “Tn” at selected epitaxial growth temperaturesaccording to the present invention.

[0031]FIGS. 3A and 3B are schematic sectional views of exemplary currentconfinement structures (Embodiment 1 and Embodiment 2, respectively)according to the present invention.

[0032]FIG. 4 is a graph of maximum optical output power as a function ofseveral device examples which have different values of the leakagecurrent path width “Tn”, according to the present invention.

[0033]FIGS. 5A to 5D are vertical sectional views sequentially showing amethod for fabricating the buried semiconductor laser device ofEmbodiment 1.

[0034]FIGS. 6A to 6D are vertical sectional views sequentially showing amethod for fabricating the buried semiconductor laser device ofEmbodiment 2.

PREFERRED EMBODIMENTS OF THE INVENTION

[0035] Referring first to FIGS. 3A and 3B, the inventors define theleakage current path widths for two basic types of buried semiconductorlaser devices, herein referred to as “Embodiment 1” and “Embodiment 2”.FIG. 3A shows a buried semiconductor device which is built upon ann-type substrate and which has two-layer current-confinement structuredisposed on either side of a light-guiding mesa ridge (Embodiment 1).The mesa ridge has an n-type bottom cladding layer formed over then-type substrate, an active layer formed over the bottom cladding layer,and a p-type top cladding layer formed over the active layer. The activelayer generally comprises two or more sub-layers. When a voltage isapplied between the top and bottom electrodes of the laser to place thedevice in an operating (lasing) state, a leakage current path is formedbetween the top and bottom electrodes which allows current to flowaround the active layers. The path is indicated in FIG. 3A as the “TnLeakage Current Path.” This current originates from the top electrode,flows substantially vertically through the p-type blocking layer alongthe side face of the buried ridge, bypassing around the active layers,and terminates into the n-type cladding layer below the active layers,as shown in FIG. 3A. We define the effective path width Tn of thisleakage current path as being the shortest distance between the sideface of the active layers and the outer current blocking layer, which isthe n-type current blocking layer. The distance Tn is a measure of theeffective cross-sectional width through which the leakage (i.e.,parasitic) current flows. With a lasing voltage applied, the amount ofleakage current increases with the value of width Tn.

[0036]FIG. 3B shows a buried semiconductor device which is built upon ap-type substrate and which has a three-layer current-confinementstructure disposed on either side of a light-guiding mesa ridge(Embodiment 2). The mesa ridge has a p-type bottom cladding layer formedover the p-type substrate, an active layer formed over the bottomcladding layer, and an n-type top cladding layer formed over the activelayer. The active layer generally comprises two or more sub-layers. Thecurrent confinement structure comprises a p-type separation layer formedover the p-type substrate and p-type bottom cladding layer, an n-typecurrent blocking layer formed over the p-type separation layer, and ap-type current blocking layer formed over the n-type current blockinglayer. When a voltage is applied between the top and bottom electrodesof the laser to place the device in an operating (lasing) state, aleakage current path is formed between the top and bottom electrodeswhich allows current to flow around the active layers. The path isindicated in FIG. 3B as the “Tp Leakage Current Path.” This currentoriginates from the bottom electrode, flows through the substrate to thep-type separation layer along the side face of the buried ridge,bypassing around the active layers, and terminating into a second uppern-type cladding layer (not shown in FIG. 3B). This second uppern-cladding layer is formed above the top current confinement layer andthe top cladding layer, both of which are shown in FIG. 3. The leakagecurrent has to pass through a constricted region located between theside face of active layer and the n-type current blocking layer. Wedefine the effective path width Tp of this leakage current path as beingthe cross-sectional width of this constricted region, which issubstantially equal to the shortest distance between the side face ofthe active layer and the n-type current blocking layer. The distance Tpis a measure of the effective cross-sectional width through which theleakage (i.e., parasitic) current flows. With a lasing voltage applied,the amount of leakage current increases with the value of width Tp.

[0037] Based on a number of experiments conducted by the presentinventors, the present inventors have discovered that, during thefabrication of the buried semiconductor laser device on an n-typesubstrate (Embodiment 1), the leakage current path width Tn varies with,and can be controlled by, the molar ratio of a group V element gas withrespect to a group III element gas contained in a raw material gas usedto form the p-type current blocking layer. The present inventors havefurther discovered that, during the fabrication of the buriedsemiconductor laser device on a p-type substrate (Embodiment 2), theleakage current path width Tp varies with, and can be controlled by, themolar ratios used to form the p-type separation layer and the n-typecurrent blocking layer. The present inventors have further discoveredthe most suitable leakage current path widths Tn and Tp, and the molarratios for realizing the most suitable widths Tn and Tp, as described ingreater detail below.

[0038] Before proceeding to the Experimental Sets, the inventors wish todefine some terms used herein. As is known in the art, “n-type” and“p-type” are conductivity types of a semiconductor substrate or layer.As is known in the art, a “n-type” semiconductor substrate or layer isformed when the number of n-type dopant atoms exceeds the number ofp-type atoms in the substrate or layer. A “p-type” semiconductorsubstrate or layer is formed when the number of p-type dopant atomsexceeds the number of n-type atoms in the substrate or layer. Normally,one does not deliberately add both p-type and n-type dopant atoms to thesame substrate or layer, since the existences of both types of dopantstends to lower the mobility of the charge-carrying carriers (e.g, holesand electrons). In addition, it is known in the art to refer to n-typeand p-type materials as being opposites of one another, since theysupport carriers of opposite charge.

[0039] In addition, the inventors have divided each side of the mesastructure into two sections: a side surface which extends from the topof the mesa down to a dividing line which lies below the active layerand usually well into the bottom cladding layer, and a skirt surfacewhich extends from this dividing line to the flat section of thesubstrate's top surface. Although not necessary, one may set thedividing line at the point where the slope of the surface makes a45-degree angle with respect to the flat surface of the substrate. Inany case, the side surface of the mesa reaches down to at least aportion of the active layer (preferably all or a substantial portion ofthe active layer) such that the active layer has at least one side whichis exposed at the side face.

[0040] Finally, all of the growth temperatures described herein andindicated in the claims are measured at the growth surface of thesubstrate. This temperature may be different from the temperature at theback surface (non-growth surface) of the substrate.

[0041] Experiment Set #1

[0042] In this Experiment Set, the variation of the leakage current pathwidth Tn in the laser structure of FIG. 3A (Embodiment 1) was measuredwhile the molar ratios of the group V element gas with respect to thegroup III element gas were changed. The study of these variations wereconducted at the following five MOCVD growth temperatures: 610° C., 640°C., 650° C., 670° C., and 700° C., as measured at the surface of thesubstrate where the epitaxial growth is occurring. The results are shownin a graph of FIG. 2A.

[0043] As shown therein, the leakage current path width Tn is regulated(i.e., controlled) at each growth temperature by the molar ratio of thegroup V element gas with respect to the group III element for formingthe p-type current blocking layer. The value of Tn increases as themolar ratio increases, for each of the growth temperatures studied.

[0044] When the molar ratio of the group V element to the group IIIelement is changed, the inventors have found that the distance and theperiod of time in which one element (for example, the group III element)of the two elements combines with the other element (for example, thegroup V element) and moves on the mesa structure to form the film isvaried. Increasing the molar ratio reduces the migration distance of thetwo elements and promotes the crawling-up of the current blocking layeralong the side faces of mesa ridge structure, which in turn increasesthe leakage current path width Tn. Reducing the molar ratio increasesthe migration distance of the two elements, which suppresses thecrawling-up of the current blocking layer, and thereby reduces theleakage current path width. Accordingly, the degree of the crawling-upof the p-type current blocking layer along the mesa ridge structure ofthe device of FIG. 3A can be changed. This same behavior occurs duringthe formation of the p-type separation layer formed on the mesa ridgestructure of the device shown in FIG. 3B, and for growing the n-typecurrent blocking layers since the presence of minute quantities ofdopant atoms (either p-type or n-type), does not substantially affectthe migration process of the principal semiconductor atoms used to formthe layers.

[0045] As a practical example, when the space around the mesa structureoverlying the n-substrate is filled for the device of FIG. 3A(Embodiment 2), the supply molar ratio is adjusted to be 20 or more tofreely change the leakage current path width Tn, thereby forming apreferable buried structure, as shown in FIG. 3A.

[0046] As another practical example, when the space around the mesastructure overlying the p-substrate is filled for the device of FIG. 3B(Embodiment 2), the supply molar ratios for forming the p-typeseparation layer (first layer) and for forming the n-type currentblocking layer (second layer) are adjusted be to 20 or more and 200 orless, respectively, thereby forming a preferable buried structure, asshown in FIG. 3B.

[0047] In summary, a larger molar ratio reduces the migration distanceand promotes the crawling-up of the current blocking layer (FIG. 3A) orthe separation layer (FIG. 3B) along the mesa ridge structure, therebyincreasing the leakage current path width. On the other hand, a smallermolar ratio increases the migration distance and suppresses thecrawling-up of the layer, thereby reducing the leakage current pathwidth.

[0048] While the results shown in FIG. 2A are for the width Tn in thedevice of FIG. 3A, it is expected that the results for the width Tp inthe device of FIG. 3B as a function of molar ratio will be substantiallythe same as those shown in FIG. 2A. The smaller number of studies thatthe inventors have conducted on the structure of FIG. 3B have providedresults for Tp as a function of molar ratio and growth temperature whichare substantially the same as the results for Tn.

[0049] Selected data points in FIG. 2A have been re-plotted in FIG. 2Bto show the dependence of Tn upon growth temperature for two selectedvalues of molar ratio (79 and 158). The results generally show a linearbehavior. The figure includes a line for the molar ratio 100, which isbased on other data points and the interpolated of the data obtained atthe molar rations of 79 and 158. The same results are expected for Tp.The studies that the inventors have conducted on the structure of FIG.3B have provided results for Tp as a function of molar ratio and growthtemperature which are substantially the same as the results for Tn.

[0050] In addition, the data of FIG. 2A has been used to constructcurves of constant Tn (and Tp) plotted in the two-dimensional domain ofgrowth temperature and molar ratio. Shown are eight curves of constantTn (and Tp) at 0.15 μm, 0.20 μm, 0.25 μm, 0.30 μm, 0.35 μm, 0.40 μm,0.50 μm, and 0.60 μm. These curves are contained within the sub-domainwhich ranges from 610° C. to 700° C. in growth temperature (T), and 50to 500 in V/III molar ratio (MR). The curve at 0.60 μm spans from MR=110at T=610° C. to MR=500 at T=640° C. The curve may be mathematicallydescribed as MR=(13.0·T−7,820), and equivalently as T=(MR+7,820)/13.0.This curve forms the upper bound of MR for growth temperatures between610° C. and 640° C. The remainder of the upper bound of MR for growthtemperatures between 640° C. and 700° C. is set at 500. At the otherextreme, the curve at 0.15 μm spans from MR=50 at T=650° C. to MR=95 atT=680° C. The curve may be mathematically described as MR=(1.5·T−925),and equivalently as T=(MR+925)/1.5. This curve forms the lower bound ofMR for growth temperatures between 650° C. and 680° C. An additionalpart of the lower bound of MR is formed by a constant value of MR=50 forgrowth temperatures between 640° C. and 650° C., and the relationshipMR=(0.833·T−483) for growth temperatures between 610° C. and 640° C.,the latter relationship being mathematically equivalent toT=(MR+483)/0.833. The remainder of the lower bound of MR is formed by asegment of the Tn=0.20 μm curve, which is described below in greaterdetail.

[0051] The Tn, Tp=0.50 μm curve has a first segment which spans fromMR=60 at T=610°C. to MR=300 at T=640° C., and a second segment whichspans from the latter point to MR=500 at T=650° C. The first segment ofthis curve may be mathematically described as MR=(8.0·T−4,820), andequivalently as T=(MR+4,820)/8. The second segment of this curve may bemathematically described as MR=(20.0·T−12,500), and equivalently asT=(MR+12,500)/20.

[0052] The Tn,Tp=0.40 μm curve has a first segment which spans fromMR=25 at T=610° C. to MR=100 at T=640° C., and a second segment whichspans from the latter point to MR=320 at T=650° C. The first segment ofthis curve may be mathematically described as MR=(2.5·T−1,500), andequivalently as T=(MR+1,500)/2.5. The second segment of this curve maybe mathematically described as MR=(22.0·T−13,980), and equivalently asT=(MR+13,980)/22.

[0053] The Tn,Tp=0.35 μm curve has a first segment which spans fromMR=50 at T=640° C. to MR=240 at T=650° C., and a second segment whichspans from the latter point to MR=460 at T=670° C. The first segment ofthis curve may be mathematically described as MR=(19.0·T−12,110), andequivalently as T=(MR+12,110)/19. The second segment of this curve maybe mathematically described as MR=(11.0·T−6,910), and equivalently asT=(MR+6,910)/11.

[0054] The Tn,Tp=0.30 μm curve spans from MR=160 at T=650° C. to MR=260at T=670° C. The curve may be mathematically described asMR=(5.0·T−3,090), and equivalently as T=(MR+3,090)/5.

[0055] The Tn,Tp=0.25 μm curve spans from MR=120 at T=650° C. to MR=160at T=670° C. The curve may be mathematically described asMR=(2.0·T−1,180), and equivalently as T=(MR+1,180)/2.

[0056] The Tn,Tp=0.20 μm curve has a first segment which spans fromMR=80 at T=650° C. to MR=120 at T=670° C., and a second segment whichspans from the latter point to MR=320 at T=700° C. The first segment ofthis curve may be mathematically described as MR=(2.0·T−1,220), andequivalently as T=(MR+1,220)/2. The second segment of this curve may bemathematically described as MR=(6.67·T−4,349), and equivalently asT=(MR+4,349)/6.67. The second segment forms the lower bound of MR fortemperatures between 680° C. and 700° C.

[0057] For ready reference, Table I below lists the values of MR at theendpoints of the segments for the above curves. TABLE I Tn, Tp (μm) MR0.15 0.20 0.25 0.30 0.35 0.40 0.50 0.60 GROWTH TEMP (° C.) 610 25 60 110640  50 100 300 500 650 50 80 120 160 240 320 500 670 80 120 160 260 460680 95 700 320

[0058] Experimental Set #2

[0059] In this Example, the relationship between the leakage currentpath width Tn for the device shown in FIG. 3A (Embodiment 1) and thelaser output power was studied by constructing a number of seven examplelasers with different values of the leakage current path width Tn andmeasuring the resulting peak output power. The cavity length of eachexample laser was the same, at a value of 1300 μm, and the constructionof the example laser is described near the end of the text. The resultsare shown in FIG. 4. A peak output power level of 360 mW was obtained ata value of Tn approximately equal to 0.30 μm. From interpolation of thedata, values of Tn from 0.25 μm to 0.35 μm are expected to provideoutput power level that are within 2% of the peak value, and values ofTn from 0.2 μm to 0.4 μm are expected to provide output power level thatare within 5% of the peak value. Accordingly, preferred embodiments growthe first current-confinement layer with values of T and MR which lie ator between the curves for 0.20 μm and 0.40 μm in FIG. 2C. These curvegenerally encompass a preferred growth region as indicated in FIG. 2C,which spans growth temperatures from 640° C. to 670° C. and molar ratios(MR) from 80 to 320. Further preferred embodiments grow the firstcurrent-confinement layer with values of T and MR which lie at orbetween the curves for 0.25 μm and 0.35 μm in FIG. 2C.

[0060] Selection of Molar Ratios

[0061] As can be seen from FIG. 2A, to achieve a value of Tn in thisrange at a growth temperature of 650° C., the molar ratio used forforming the p-type current blocking layer overlying the n-substrate inthe device shown in FIG. 3A (Embodiment 1) should be adjusted to bebetween 50 and 500, inclusive. When the molar ratio is below 50 at thisgrowth temperature, the reproducibility of forming the currentconfinement structure having the specified leakage current path width Tnis deteriorated, and molar ratios below 50 at this temperature do notregulate or control the leakage current path widths very well. On theother hand, when the molar ratio exceeds 500 at this growth temperature,the leakage current path width Tn is increased to a value of over 0.5μm, thereby providing a large leakage current. In more preferredembodiments of the device shown in FIG. 3A, a value for Tn in the rangefrom 0.2 μm to 0.4 μm is used to further reduce the leakage current.This range can be achieved at a growth temperature of 650° C. with amolar ratio between 80 and 320, inclusive, for forming the p-typecurrent blocking layer. In further preferred embodiments, Tn is set inthe range from 0.25 μm to 0.35 μm, which can be achieved at a growthrate of 650° C. with a molar ratio between 120 and 240, inclusive.

[0062] The n-type current blocking layer in the device shown in FIG. 3Ais preferably grown at a growth temperature and molar ratio whichprovides the same or larger degree of creep up the mesa ridge than thatused in growing the p-type current blocking layer. Referring to FIG. 2C,this corresponds to selecting a growth temperature (T) and molar ratio(MR) which lie on a Tn,Tp curve which has a larger value than that usedto grow the p-type current blocking (confinement) layer. Accordingly,when the n-type current blocking ratio is grown at the same temperatureas the p-type current blocking layer, the same or larger molar ratio ispreferably used for forming the n-type current blocking layer.Alternatively, when the n-type current blocking layer is grown with thesame molar ratio as the p-type current blocking layer, the same or lowergrowth temperature is preferably used for forming the n-type currentblocking layer. Thereby, the growth rate from the side of the mesastructure becomes larger during the formation of the n-type currentblocking layer, which suppresses the formation of structural defects,such as hollows, trenches, and crystal dislocations.

[0063] Similarly, to achieve a value of Tp in the range from 0.15 μm to0.6 μm at a growth temperature of 650° C. for the device of FIG. 3B(Embodiment 2), the molar ratio used for forming the p-type separationlayer overlying the p-substrate should be adjusted to be between 50 and500, inclusive. When the molar ratio is below 50 at this growthtemperature, the reproducibility of the current confinement structurebecomes poor, and the value of leakage current path width Tp is notcontrolled very well. On the other hand, when the molar ratio exceeds500 at this growth temperature, the leakage current path width Tp isincreased to a value of over 0.5 μm, thereby providing a large leakagecurrent. In more preferred embodiments of the device shown in FIG. 3B, avalue for Tp in the range from 0.2 μm to 0.4 μm is used to furtherreduce the leakage current. This range can be achieved at a growthtemperature of 650° C. with a molar ratio between 80 and 320, inclusive,for forming the p-type separation layer. In further preferredembodiments, Tp is set in the range from 0.25 μm to 0.35 μm, which canbe achieved at a growth rate of 650° C. with a molar ratio between 120and 240, inclusive.

[0064] The n-type current blocking layer of the device in FIG. 3B isthen grown over the p-type separation layer, preferably at a growthtemperature and molar ratio which provides less creep up the mesa ridgethan that achieved in growing the p-type separation layer. Referring toFIG. 2C, this corresponds to selecting a growth temperature (T) andmolar ratio (MR) which lie on a Tn,Tp curve which has a smaller valuethan that used to grow the p-type separation layer. Accordingly, whenthe n-type current blocking ratio is grown at the same temperature asthe p-type separation layer, a lower molar ratio is preferably used forforming the n-type current blocking layer. Alternatively, when then-type current blocking layer is grown with the same molar ratio as thep-type separation layer, a higher growth temperature is preferably usedfor forming the n-type current blocking layer. At an exemplary growthtemperature of 650° C., the molar ratio for forming the n-type currentblocking layer is preferably between 30 and 80, inclusive. When thismolar ratio is below 30, structural defects are formed firstly on thesurface of the n-type current blocking layer and then on the surface ofthe p-type current blocking layer that is subsequently formed. At theother extreme, when the molar ratio is larger than 80, the n-typecurrent blocking layer grows not only from the top surface of the skirtof the mesa structure but also from the side surface thereof such thatthe n-type current blocking layer makes contact with the n-type topcladding layer of the mesa ridge structure, or with the n-type contactlayer which is formed at a later step. The contacting of the n-typecurrent blocking layer with either or both of these other n-type layersis undesirable because it creates another leakage current path.

[0065] The p-type current blocking layer of the device in FIG. 3B(Embodiment 2) is preferably grown at a growth temperature and molarratio which provides the same or larger degree of creep up the mesaridge than that used in growing the n-type current blocking layer.Accordingly, when the p-type current blocking ratio is grown at the sametemperature as the n-type current blocking layer, the same or largermolar ratio is preferably used for forming the p-type current blockinglayer. Alternatively, when the p-type current blocking layer is grownwith the same molar ratio as the n-type current blocking layer, the sameor lower growth temperature is preferably used for forming the p-typecurrent blocking layer. Thereby, the growth rate of the p-type currentblocking layer from the side surface of the mesa structure becomeslarger to suppress the structural defects.

[0066] The methods according to the present invention (as exemplified inFIGS. 3A and 3B, respectively) can be applied without limitation to anyconfiguration including the semiconductor substrate, the bottom claddinglayer, the active layer, the top cladding layer and the current blockinglayer. In addition, the shape of the mesa structure is not restricted solong as the mesa structure is the buried one.

[0067] The buried semiconductor laser device formed on the n-type or thep-type substrate in accordance with the present invention has thereduced leakage current and includes the current confinement structurehaving substantially no structural defects. Accordingly, the buriedsemiconductor laser device with the higher output can be fabricated withexcellent reproducibility and/or higher yield having the better outputcharacteristics such as the threshold current and the emissionefficiency, and with good linearity of the current-voltagecharacteristic in the lasing regime.

[0068] In the semiconductor laser devices formed on the p-type substrateand the n-type substrate in accordance with the present invention, theleakage current path widths Tp and Tn have the following ranges ofvalues:

[0069] 0.15 μm<Tp<0.6 μm

[0070] 0.15 μm<Tn<0.6 μm.

[0071] The more preferred range of values is:

[0072] 0.2 μm≦Tp≦0.4 μm

[0073] 0.2 μm≦Tn≦0.4 μm,

[0074] and the most preferred range of values is:

[0075] 0.25 μm≦Tp≦0.35 μm

[0076] 0.25 μm≦Tn≦0.35 μm.

[0077] These ranges realize the buried semiconductor laser device havingthe above excellent characteristics.

[0078] The configuration of a buried semiconductor laser device inaccordance with embodiments of the present invention will now bedescribed referring to the annexed drawings.

[0079] Examples of the Two-Layer Current Confinement Structure (FIG. 3A)

[0080] We first describe the general structure of the example devicesused to generate the data points shown in FIG. 4, and thereafter morefully describe devices in detail. These devices use the structure shownin FIG. 3A (Embodiment 1).

[0081] As shown in FIG. 5A, a multi-layer structure including an n-typeInP bottom cladding layer 52, a bottom GRIN-SCH layer 53, an activelayer 54 having a strained multiple quantum well structure, a topGRIN-SCH layer 55 and a p-type InP top cladding layer 56 sequentiallydeposited is formed on an n-type InP substrate 51 by the MOCVD methodusing conventional epitaxial growth conditions for growing these mesalayers.

[0082] Then, as shown in FIG. 5B, an etching mask 57 formed by a siliconnitride (SiN_(x)) film is deposited on the top cladding layer 56, and,by using the etching mask 57, the top cladding layer 56, the topGRIN-SCH layer 55, the active layer 54, the bottom GRIN-SCH layer 53 andthe top of the bottom cladding layer 52 in the multi-layered structureare etched with an etching solution 57, thereby forming a mesa structure58 having the undercut on the bottom surface of the etching mask 57.

[0083] Then, as shown in FIG. 5C, a p-type InP current blocking layer 59having a thickness of 1 μm is deposited on the side surface 58 a and thetop skirt surface 58 b of the mesa structure 58 by using the MOCVDmethod at a growth temperature of 650° C. A raw material gas usedtherein contains a group V element gas and a group III element gas in amolar ratio (V/III) between 50 and 500, inclusive. The space around themesa structure 58 is further filled by subsequently depositing an n-typeInP current blocking layer 60 having a thickness of 1 μm to 2 μm on thep-type InP current blocking layer 59 at the molar ratio larger than thatused for forming the p-type InP current blocking layer 59. The growthtemperature is maintained substantially at 650° C.

[0084] As an example, trimethylindium (TMIn), phosphine (PH₃) anddiethyl zinc (DEZn) may be used as the group III element gas, the groupV element gas and a doping gas, respectively, for forming the p-type InPcurrent blocking layer 59; the same group III and V element gases asthose for forming the p-type InP current blocking layer 59 and hydrogensulfide (H₂S) as the doping gas may be used for forming the n-type InPcurrent blocking layer 60.

[0085] Next, as shown in FIG. 5D, a p-type InP top cladding layer 61 anda p-type GaInAsP cap layer 62 are sequentially formed on the mesastructure 58 and the current blocking layers 59, 60 by using the MOCVDmethod.

[0086] After the formation of a p-side electrode 63 on the p-typeGaInAsP cap layer 62, the bottom surface of the n-type InP substrate 51is polished to control the total thickness of the substrate to about 0.1mm (100 μm), and an n-side electrode 64 is formed on the polishedsurface.

Example 1

[0087] Example 1 uses the above fabrication steps with the p-type InPcurrent blocking layer 59 and the n-type InP current blocking layer 60being formed at the growth temperature of 650° C. and at the molarratios of 80 and 158, respectively, for filling the space around themesa structure 58. After the top cladding layer 61 and the cap layer 62were sequentially formed, the two electrodes 63, 64 were formed thereonand on the bottom surface of the substrate 51. Then, the obtainedstructure was cleaved, a low reflection film and a high reflection filmwere formed on respective ones of the cleaved surfaces, therebyfabricating a semiconductor laser device of Example 1 having an emissionwavelength range from 1.28 μm to 1.63 μm.

[0088] As shown in Table 2, the leakage current path width “Tn” observedwith an electron microscope was 0.19 μm. TABLE 2 Leakage Molar RatioMolar Ratio Current of p-type of n-type Path Conduct- Current CurrentWidth ivity of Growth Blocking Blocking “Tn” Substrate Temp. Layer Layer(μm) Example 1 n 650° C.  80 158 0.19 Example 2 n 650° C.  80 158 0.21Example 3 n 650° C. 316 158 0.41 Example 4 n 650° C. 316 158 0.45Example 5 n 650° C. 316 158 0.45 Example 6 n 650° C. 158 158 0.30(highest output) Comp. n 650° C.  20 158 0.10 Example 1 Comp. n 610° C.158 158 0.65 Example 2

Examples 2 and 3

[0089] Buried semiconductor laser devices of Examples 2 and 3 having theemission wavelength range from 1.28 μm to 1.63 μm were fabricated byusing the same conditions as those of Example 1 except that the molarratios of the raw material gas for forming the p-type InP currentblocking layer 59 and the n-type InP current blocking layer 60 wereshown in Table 2, which were different from those of Example 1. Theleakage current path widths “Tn” thereof were 0.21 μm and 0.41 μm,respectively, as shown in Table 2.

Examples 4 and 5

[0090] Buried semiconductor laser devices of Examples 4 and 5 having theemission wavelength range from 1.28 μm to 1.63 μm were fabricated byusing the same conditions as those of Example 1 except that the molarratios of the raw material gas for forming the p-type InP currentblocking layer 59 and the n-type InP current blocking layer 60 were 316and 158, respectively. The leakage current path width “Tn” of eachdevice was 0.45 μm. The devices did, however, have slightly differentoutput power levels (350 mW versus 340 mW), which can be due variationsin the cavity lengths and the reflectivities of the reflection filmsformed on the laser facets.

Example 6

[0091] Buried semiconductor laser devices of Example 6 having anemission wavelength in the range of 1.28 μm to 1.63 μm was fabricated byusing the same conditions as those of Example 1 except that the molarratios of the raw material gas for forming the p-type InP currentblocking layer 59 and the n-type InP current blocking layer 60 were eachset at 158. The leakage current path width “Tn” of the device was 0.30μm. This device had the highest output power level (360 mW) of theconstructed devices.

Comparative Examples 1 and 2

[0092] For the purpose of evaluating the fabrication methods of Examples1 to 6, buried semiconductor laser devices of Comparative Examples 1 and2 having the emission wavelength range from 1.28 μm to 1.63 μm werefabricated by using the same conditions as those of Example 1 exceptthat the molar ratios of the raw material gas for forming the p-type InPcurrent blocking layer 59 and the n-type InP current blocking layer 60were as shown in Table 2, which were out of the range specified in thepresent invention. The leakage current path widths “Tn” thereof were0.10 μm and 0.65 μm, respectively, as shown in Table 2.

[0093] In the buried semiconductor laser devices of Examples 1 to 6, theleakage current path widths “Tn” were controlled in the range from 0.19μm to 0.45 μm to have smaller leakage current.

[0094] On the other hand, in the buried semiconductor laser device ofComparative Example 1, the leakage current path width “Tn” was 0.10 μmwhich was too small, and the reproducibility of the leakage currentcharacteristic was too poor to be quantified because the molar ratio forforming the p-type current blocking layer is smaller than thatprescribed in the first invention. On the other hand, in the buriedsemiconductor laser device of Comparative Example 2, the leakage currentpath width “Tn” was 0.60 μm, which was large enough to generate a largerleakage current.

[0095] Exemplary Construction of the Three-Layer Current ConfinementStructure (FIG. 3B)

[0096] As shown in FIG. 6A, a multi-layered film is formed including ap-type InP bottom cladding layer 72, a bottom GRIN-SCH layer 73, anactive layer 74 having a strained multiple quantum well structure, a topGRIN-SCH layer 75 and an n-type InP top cladding layer 76 sequentiallyformed on a p-type InP substrate 71 by using the MOCVD method usingconventional epitaxial growth conditions for growing these mesa layers.

[0097] After an etching mask 77 made of a silicon oxide film is formedon the top cladding layer 76 as shown in FIG. 6B, the top cladding layer76, the top GRIN-SCH layer 75, the active layer 74, the bottom GRIN-SCHlayer 73 and the top portion of the bottom cladding layer 72 are etchedwith an etching solution by using the etching mask 77, thereby forming amesa structure 78 having the undercut on the bottom surface of theetching mask 77.

[0098] Then, as shown in FIG. 6C, a p-type InP separation layer 79 isdeposited on the side surface 78 a and the top skirt surface 78 b of themesa structure 78 by using the MOCVD method by using a raw material gascontaining a group V element gas and a group III element gas in a molarratio (V/III) between 50 and 500 inclusive. Further, an n-type InPcurrent blocking layer 80 and a p-type InP current blocking layer 81 areformed by using raw material gases having molar ratios (V/III) between30 and 80 inclusive and between 50 and 500 inclusive, respectively, tofill the space around the mesa structure 78.

[0099] For example, trimethylindium (TMIn), phosphine (PH₃) anddiethylzinc (DEZn) may be used as the group III element gas, the group Velement gas and a doping gas, respectively, for forming the p-type InPseparation layer 79 and the p-type current blocking layer 81.

[0100] For example, the same group III and V element gases as those forforming the p-type InP separation layer 79 and the p-type currentblocking layer 81, and hydrogen sulfide (H₂S) as the doping gas may beused for forming the n-type InP current blocking layer 80.

[0101] Then, as shown in FIG. 6D, an n-type InP top cladding layer 82and an n-type GalnAsP cap layer 83 are sequentially formed on the mesastructure 78, the p-type separation layer 79 and the current blockinglayer 81 by using the MOCVD method.

[0102] After the formation of an n-side electrode 84 on the n-typeGaInAsP cap layer 83, the bottom surface of the n-type InP substrate 71is polished to control the total thickness of the substrate to about 0.1μm, and a p-side electrode 85 is formed on the polished surface.

[0103] As mentioned above, the dependence of the width Tp on the molarratio and growth temperature for the device of FIG. 3B is expected to besubstantially the same as the dependence of the width Tn on molar ratioand growth temperature for the device of FIG. 3A since the degree ofcrawl up is not substantially affected by the minute quantities ofdopant atoms (either p-type or n-type). The studies that the inventorshave conducted on the structure of FIG. 3B have provided results for Tpas a function of molar ratio and growth temperature which aresubstantially the same as the results for Tn.

[0104] General Definition of Molar Ratio

[0105] The precursor gases used today to construct III-V semiconductordevices contain group III and group V atoms in mono-atomic form. That isto say that each molecule of a precursor gas only has one atom from thethird column of the periodic table, or only one atom from the fifthcolumn of the periodic table. Thus, in this case, to compute the molarquantity of the group V element gas, one multiplies the molar quantityof each group V precursor gas by 1, and sums the quantities together. Ina similar manner, to compute the molar quantity of the group III elementgas, one multiplies the molar quantity of each group III precursor gasby 1, and sums the quantities together. The molar ratio is then thedivision of the two summed quantities.

[0106] In the event that the industry develops multi-atomic forms ofprecursor gases (that is, two or more group V atoms per molecule, or twoor more group III atoms per molecule), the above computation of themolar ratio is modified as follows. For each multi-atomic precursor gas,one multiplies the molar quantity of the gas by the number of group V orgroup III atoms per molecule, before summing the quantities together.

[0107] In working with MOCVD equipment, the gases are generally fed intothe reaction chamber in rates that can be quantified in terms of molesper liter. The above computation for the molar ratio may be carrier outin terms of moles per liter. The liter dimension is common to all gases,and cancels from the ratio calculation.

[0108] Since the above embodiment is described only for examples, thepresent invention is not limited to the above embodiment and variousmodifications or alterations can be easily made therefrom by thoseskilled in the art without departing from the scope of the presentinvention.

What is claimed is:
 1. A method for fabricating a buried semiconductorlaser device comprising the steps of: forming a mesa structure includinga bottom cladding layer, an active layer and an upper cladding layeroverlying a semiconductor substrate, the mesa structure having at leastone side surface with the active layer having an exposed side thereat;and growing a first current-confinement layer on the mesa's at least oneside surface, the first current-confinement layer comprising asemiconductor material and having a first conductivity type; growing asecond current-confinement layer above at least a portion of the firstcurrent-confinement layer, the second current-confinement layercomprising a semiconductor material and having a second conductivitytype which is opposite to the first conductivity type; and wherein thefirst current-confinement layer is grown at a temperature ranging from610° C. to 700° C. using a raw material gas containing a group V elementgas and a group III element gas at a molar ratio of the group V elementgas with respect to the group III element gas between 50 and 500,inclusive.
 2. The method as defined in claim 1 wherein the firstcurrent-confinement layer is grown at a temperature ranging from 640° C.to 670° C. using a raw material gas containing a group V element gas anda group III element gas at a molar ratio of the group V element gas withrespect to the group III element gas between 80 and
 320. 3. The methodas defined in claim 1, wherein the second current-confinement layer isformed using a raw material gas having a molar ratio different from themolar ratio used in the formation of the first current-confinementlayer.
 4. The method as defined in claim 3, wherein the firstcurrent-confinement layer and the second current-confinement layer aregrown at substantially the same temperature.
 5. The method as defined inclaim 3, wherein the molar ratio used for the formation of the secondcurrent confinement layer is larger than that used for the formation ofthe first current confinement layer.
 6. The method as defined in claim3, wherein the molar ratio used for the formation of the second currentconfinement layer is smaller than that used for the formation of thefirst current confinement layer.
 7. The method as defined in claim 6,wherein the molar ratio used for the formation of the second currentconfinement layer is in a range from 30 to 80, inclusive.
 8. The methodas defined in claim 7, wherein the molar ratio used for the formation ofthe first current confinement layer is in a range from 80 to 320,inclusive.
 9. The method as defined in claim 1, wherein the firstcurrent-confinement layer is formed at a first growth temperature, andwherein the second current-confinement layer is formed at a secondgrowth temperature which is different from the first growth temperature.10. The method as defined in claim 9, wherein the firstcurrent-confinement layer and the second current-confinement layer aregrown with molar ratios which are substantially the same.
 11. The methodas defined in claim 9, wherein second growth temperature is lower thanthe first growth temperature.
 12. The method as defined in claim 9,wherein second growth temperature is higher than the first growthtemperature.
 13. The method as defined in claim 1 wherein the substratehas an n-type conductivity, wherein the first conductivity type isp-type, and wherein the second conductivity type is n-type.
 14. Themethod as defined in claim 1 wherein the substrate has a p-typeconductivity, wherein the first conductivity type is p-type, and whereinthe second conductivity type is n-type.
 15. The method as defined inclaim 3 further comprising the step of growing a thirdcurrent-confinement layer above the second current-confinement layer,the second current-confinement layer comprising semiconductor materialand having the first conductivity type, wherein thirdcurrent-confinement layer is formed with a raw material gas having amolar ratio different from the molar ratio used in the formation of thefirst current-confinement layer.
 16. The method as defined in claim 15,wherein the molar ratio used for the formation of the third currentconfinement layer is larger than that used for the formation of thefirst current confinement layer.
 17. The method as defined in claim 1wherein the temperature at which the first current-confinement layer isgrown is represented by a temperature T; wherein the firstcurrent-confinement layer is grown at a molar ratio of the group Velement gas with respect to the group III element gas equal to orgreater than the quantity (1.5·T−925) when the growth temperature isbetween 650° C. and 680° C., inclusive; and wherein the firstcurrent-confinement layer is grown at a molar ratio of the group Velement gas with respect to the group III element gas equal to orgreater than the quantity (6.67·T−4,349) when the growth temperature isgreater than 680° C. and less than or equal to 700° C.
 18. The method asdefined in claim 17 wherein the first current-confinement layer is grownat a molar ratio of the group V element gas with respect to the groupIII element gas equal to or greater than the quantity (2.0·T−1,220) whenthe growth temperature is between 650° C. and 670° C., inclusive; andwherein the first current-confinement layer is grown at a molar ratio ofthe group V element gas with respect to the group III element gas equalto or greater than the quantity (6.67·T−4,349) when the growthtemperature is greater than 670° C. and less than 680° C..
 19. Themethod as defined in claim 17 wherein the first current-confinementlayer is grown at a molar ratio of the group V element gas with respectto the group III element gas equal to or greater than the quantity(2.0·T−1,180) when the growth temperature is between 650° C. and 670°0C., inclusive.
 20. The method as defined in claim 1 wherein thetemperature at which the first current-confinement layer is grown isrepresented by a temperature T; wherein the first current-confinementlayer is grown at a molar ratio of the group V element gas with respectto the group III element gas which is less than or equal to the quantity(13.0·T−7,820) when the growth temperature is between 610° C. and 640°C., inclusive.
 21. The method as defined in claim 20 wherein the firstcurrent-confinement layer is grown at a molar ratio of the group Velement gas with respect to the group III element gas which is less thanor equal to the quantity (8.0·T−4,820) when the growth temperature isbetween 610° C. and 640° C., inclusive; and wherein the firstcurrent-confinement layer is grown at a molar ratio of the group Velement gas with respect to the group III element gas which is less thanor equal to the quantity (20.0·T−12,500) when the growth temperature isgreater than 640° C. and less than or equal to 650° C.
 22. The method asdefined in claim 20 wherein the first current-confinement layer is grownat a molar ratio of the group V element gas with respect to the groupIII element gas which is less than or equal to the quantity(2.5·T−1,500) when the growth temperature is between 610° C. and 640°C., inclusive; and wherein the first current-confinement layer is grownat a molar ratio of the group V element gas with respect to the groupIII element gas which is less than or equal to the quantity(22.0·T−13,980) when the growth temperature is greater than 640° C. andless than or equal to 650° C.
 23. The method as defined in claim 20wherein the first current-confinement layer is grown at a molar ratio ofthe group V element gas with respect to the group III element gas whichis less than or equal to the quantity (19.0·T−12,110) when the growthtemperature is between 640° C. and 650 ° C., inclusive; and wherein thefirst current-confinement layer is grown at a molar ratio of the group Velement gas with respect to the group III element gas which is less thanor equal to the quantity (11.0·T−6,910) when the growth temperature isgreater than 650° C. and less than or equal to 670° C.
 24. The method asdefined in claim 23 wherein the first current-confinement layer is grownat a molar ratio of the group V element gas with respect to the groupIII element gas equal to or greater than the quantity (2.0·T−1,180) whenthe growth temperature is between 650° C. and 670° C., inclusive.
 25. Amethod for fabricating a buried semiconductor laser device comprisingthe steps of: forming a mesa structure including a bottom claddinglayer, an active layer and a top cladding layer overlying an n-typesemiconductor substrate; and forming a current confinement structure bygrowing a p-type current blocking layer and an n-type current blockinglayer on each side surface of the mesa structure and on a skirt portionextending from each side surface, the p-type current blocking layerbeing fabricated at a growth temperature ranging from 610° C. to 700° C.and by using a raw material gas containing a group III element gas and agroup V element gas at a molar ratio of the group V element gas withrespect to the group III element gas between 50 and 500, inclusive. 26.The method as defined in claim 25, wherein the molar ratio is between 80and 320, inclusive.
 27. The method as defined in claim 25, wherein then-type current blocking layer is formed by using another raw materialgas having a molar ratio different from the molar ratio used for theformation of the p-type current blocking layer.
 28. The method asdefined in claim 27, wherein the molar ratio used for the formation ofthe n-type current blocking layer is larger than that used for theformation of the p-type current blocking layer.
 29. A method forfabricating a buried semiconductor laser device comprising the steps of:forming a mesa structure including a bottom cladding layer, an activelayer and a top cladding layer overlying a p-type semiconductorsubstrate; and forming a current confinement structure by growing ap-type separation layer, an n-type current blocking layer and a p-typecurrent blocking layer on each side surface of the mesa structure and ona skirt portion extending from each side surface, the p-type separationlayer being fabricated at a growth temperature ranging from 610° C. to700° C. and by using a raw material gas containing a group III elementgas and a group V element gas at a molar ratio of the group V elementgas with respect to the group III element gas between 50 and 500,inclusive.
 30. The method as defined in claim 29, wherein the firstmolar ratio is between 50 and 500 inclusive, and the second molar ratiois between 30 and 80 inclusive.
 31. The method as defined in claim 30,wherein the first molar ratio is between 80 and 320, inclusive.
 32. Themethod as defined in claim 29, wherein the p-type separation layer, then-type current blocking layer and the p-type current blocking layer areformed by using the raw material gases having different molar ratiosamong one another.
 33. The method as defined in claim 29, wherein athird molar ratio for forming the p-type current blocking layer islarger than the first molar ratio.
 34. A buried semiconductor laserdevice formed on a semiconductor substrate having a top surface and abottom surface, the laser device comprising: a mesa structure formed atthe top surface of the substrate and having a bottom cladding layer atthe top surface of the substrate, an active layer overlaying the bottomcladding layer, and a top cladding layer overlying the active layer, themesa structure having at least one side surface with the active layerhaving an exposed side thereat; a first current-confinement layeroverlaying at least a portion of the mesa's at least one side surfaceand having a first portion disposed against the exposed side of theactive layer, the first current-confinement layer comprising asemiconductor material and having a first conductivity type; a secondcurrent-confinement layer overlaying at least a portion of the firstcurrent-confinement layer, the second current-confinement layercomprising a semiconductor material and having a second conductivitytype which is opposite to the first conductivity type; and wherein theclosest distance (Tn or Tp) between the second current-confinement layerand the active layer has a value in the range from 0.15 μm to 0.6 μm.35. The laser device as defined in claim 34 wherein the substrate has ann-type conductivity, wherein the first conductivity type is p-type, andwherein the second conductivity type is n-type.
 36. The laser device asdefined in claim 35 wherein the first current confinement layer is afirst current-blocking layer, and wherein the second current confinementlayer is a second current-blocking layer.
 37. The laser device asdefined in claim 34 wherein the substrate has a p-type conductivity,wherein the first conductivity type is p-type, and wherein the secondconductivity type is n-type.
 38. The laser device as defined in claim 37wherein the first current confinement layer is a separation layer, andwherein the second current confinement layer is a first current-blockinglayer.
 39. The laser device as defined in claim 34 wherein the closestdistance (Tn or Tp) has a value in the range from 0.2 μm to 0.4 μm,inclusive.
 40. The laser device as defined in claim 34 wherein theclosest distance (Tn or Tp) has a value in the range from 0.25 μm to0.35 μm, inclusive.